The present disclosure relates to a light application, and more specifically to a cathode structure. The cathode structure finds particular application in large area organic light emitting devices, although it will be appreciated that selected aspects may find use in related applications encountering the same issues of degradation due to current fluctuation during run-up, dimming or cycling.
An organic light emitting device (OLED) is a type of a light emitting diode that emits light in response to an applied potential. A typical OLED comprises an anode, one or more organic material layers and a cathode. Cathodes generally comprise a material having a low work function such that a relatively small voltage causes the emission of electrons. Some commonly used material include metals, such as gold, gallium, indium, manganese, tin, led, aluminum, silver, magnesium, a silver/magnesium alloy or combinations thereof. Such materials, although having a low work function, exhibit relatively low melting points and/or exhibit high degradation when exposed to oxygen or water. Anodes generally comprise a transparent material having high work function value such as indium tin oxide (ITO), tin oxide, nickel, or gold. A layer of molybdenum oxide (MoO3) may be included to reduce the overall driving voltage.
One of the layers of the OLED comprises a material having the ability to transport holes, and is referred to as the hole transport layer. Another layer typically comprises a material having the ability to transport electrons, known as the electron transport layer. This layer may also function as the luminescent material (or emission layer) or an additional independent layer may be disposed between the hole transport layer and the electron transport layer. When a voltage is applied, a current of electrons flow through the device from the cathode to the anode. The anode injects positive charges (holes) into the hole transport layer, while the cathode injects negative charges (electrons) into the electron transport layer. Electrostatic forces bring the electrons and the holes together and they recombine near the light emitting layer, which causes a drop in energy levels and an emission of radiation in the range of visible light.
Organic light emitting diodes are currently used for display applications and are planned for use in general lighting applications. An OLED device includes one or more organic light emitting layers disposed between two electrodes, e.g., a cathode and a light transmissive anode, formed on a light transmissive substrate. The organic light emitting layer emits light upon application of a voltage across the anode and cathode. Upon the application of a voltage from a voltage source, electrons are directly injected into the organic layer from the cathode, and holes are directly injected into the organic layer from the anode. The electrons and the holes travel through the organic layer until they recombine at a luminescent center. This recombination process results in the emissions of a photon, i.e., light.
Large area OLED devices typically combine many individual OLED devices on a single substrate or a combination of substrates with multiple individual OLED devices on each substrate. Applications for large area OLED devise include lighting. For most of these applications, alternating current (AC) power is most readily available. However, OLEDs have rectifying current/voltage characteristics and so are typically operated with direct current (DC) power wired with the correct polarity for light emission. In these applications, AC power is converted to DC power to operate the large area OLEDs.
However, current OLED technologies for current driven devices exhibit power control problems. Eddy current is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor. When a conductor moves relative to the field generated by a source, electromotive forces (EMFs) can be generated around loops within the conductor. These EMFs acting on the resistivity of the material generate a current around the loop, in accordance with Faraday's law of induction. These currents dissipate energy, and create a magnetic field that tends to oppose the changes in the field. Therefore, when a moving conductor experiences changes in the magnetic field generated by a stationary object, as well as when a stationary conductor encounters a varying magnetic field, an eddy current is formed. This is a problem for organic light emitting devices (OLEDs). This can induce eddy current and create degradation in the organic layer and organic/cathode interface. There may be also reduce efficiency of the device by reducing charge injection efficiency and induce quenching of light.
Furthermore, large area OLEDs may include a single device or devices that can be connected to form large area OLEDs having a large capacitance. Capacitance is the ability of a body to hold an electrical charge. It is also a measure of the amount of electric charge stored (or separated) for a given electric potential. Due to a large capacitance, current flow through the device may overshoot or reflect a large fluctuation in current. The overshoot or large fluctuation in current can cause damage to the OLED by dissociating the organic layer and/or burning the cathode structure which is typically aluminum. This can even be more detrimental for large area devices as the capacitance can increase with area.
Significant efforts have been made in selecting materials and forming modified layer structures or materials in OLEDs to achieve improved performance. Numerous OLEDs with alternative layer structures have been disclosed. For example, OLEDs have been created containing additional functional layers. Some of these new layer structures with new materials have indeed resulted in improved device performance.
Even in light of recent advances, there is a continued need to improve OLED structure by reducing the eddy current and large fluctuations in current, thereby further enhancing the performance and efficiency of an OLED for use as a light source.